And every priest standeth daily ministering and offering oftentimes the same sacrifices, which can never take away sins: But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God; From henceforth expecting till his enemies be made his footstool. For by one offering he hath perfected for ever them that are sanctified.
“The Aaronic priests never sat down in the sanctuary; they remained standing throughout the whole performance of their sacred duties (Bruce, F.F. The Epistle to the Hebrews, New International Commentary on the New Testament, Eerdmans, 1979, page 238).” They never sat because their work was never done.
(NavPress Bible study, Hebrews, NavPress, Colorado Springs, CO: 1989, pg 131)